Loop cut shortcut not working on Blender 2.8
In a comment you write


  I was in edit mode and have selected the loop cut tool, the same tool you have there in the image.


That is exactly the problem. If you have the Loop Cut tool selected in the toolbar, choosing the number of cuts with the mouse wheel doesn't work and LMB does not allow you to slide.

Try selecting the Select Box tool (or any other tool really) and then Ctrl+R will work as shown in the tutorial.